The routine transport of pollution aerosol through long distances is increasingly recognized as an important aspect of atmospheric science. Atmospheric transport on the scale of 1000 to 10,000 km is now invoked regularly to explain the results of aerosol studies in rural and remote areas. P. J. Samsonb, J. Appl. Meteorol 19, 1382 (1980); K. A. Rahn and R. J. McCaffrey, Ann. N.Y. Acad. Sci. 338, 486 (1980); R. D. Borys and K. A. Rahn, Atoms. Environ. 15, 1491 (1981); L. A. Barrie, R. M. Hoff, S. M. Daggupaty, ibid., p. 1407; C. Brosset, Ambio 5, 157 (1976).
But long-range transport has created a new set of interpretive problems. While it is relatively easy to identify pulses of transported pollution aerosols in remote areas which are otherwise clean, it is often difficult or impossible to pinpoint the source areas of these aerosols. (At distances of a few hundred kilometers or more, source areas are normally much more important than point sources).
Sheer distance can cause problems. For example, it has been extremely difficult to decide whether the important sources of pollution aerosol observed at Barrow, Alaska, are located in North America, Europe, or Asia. With air-mass trajectories from these sources being 5000 to 10,000 km or more in length and representing travel times of 5 to 10 days or more, pure meterological techniques have not led to consensus about even the continents of origin, much less particular regions within the continents. J. M. Miller, Atoms. Environ. 15, 1401 (1981); E. R. Reiter, ibid., p. 1465; D. E. Patterson and R. B. Husar, ibid., p. 1479.
The configuration of sources can also make identification difficult. In the northeastern United States, for example, where the source areas of acid aerosol and precipitation are currently in dispute, distances of transport are much shorter (1000 km or less) but the spatial pattern of sources is complex. As a result, trajectories to areas of concern such as the Adirondacks or New England often pass over several strong source areas in their last few hundred kilometers. No available transport model can reliably apportion the contributions of these sources to the final sulfate, acid, or other ubiquitous constituents of the pollution aerosol.
There is thus a need for a more direct way to identify distant sources of pollution aerosol. Such a capability would be of practical as well as scientific importance, because it could be extended ultimately to determining source areas of acid precipitation. It may cost as much as $20 billion to $100 billion to reduce emissions of sulfur dioxide in the eastern United States over the next decade; controlling the wrong sources would be a very costly error.
The present invention embodies a method to detect regional sources of pollution on a regional scale. The efforts to date in this field can only trace individual emitters over smaller distances.
Pollution aerosol contains all elements; no true tracers, or elements unique to specific source areas, exist. But it is reasonable to expect the proportions of at least some elements to vary with source area because different areas have different mixes of the major aerosol sources (combustion, industry, transportation, and so on), different mixes of fuels, fuels from different origins, different industrial bases, and different degrees of pollution control. However, the number of regional elemental signatures, the magnitude of their differences, and the elements involved cannot be predicted; they must be determined empirically.
In general, regional tracers as used in our invention are constructed and used quite differently from urban tracers. Elemental signatures used to deduce sources of urban aerosol by receptor-oriented techniques (G. E. Gordon, Environ. Sci. Technol. 14, 792 (1980).) are usually derived from either point sources or specific types of sources (automotive exhaust, for example). Regional aerosols, by contrast, are mixes of many sources and should thus resemble one another much more than signatures within an urban region should. Similarities among pollution aerosols have been recognized for years (K. A. Rahn, "The chemical compositon of the atmospheric aerosol," Technical Report, Graduate School of Oceanography, University of Rhode Island (1976).), and many have doubted whether useful regional differences could be found. We have determined that characteristic regional signatures do exist, many of which are very different from one another.
The two keys to deriving regional signatures are finding the right elements and handling the data with the appropriate statistical techniques. The "marker-element" approach sometimes used in urban studies (where the contribution of a source is evaluated by a single element) cannot be used with regional signatures because of their great similarities. The opposite approach, constructing signatures from all available elements, is practiced in some urban studies but addes too much noise to regional pollution signatures. The best approach seems to be a compromise-limit regional signatures to those few elements with the greatest tracer power.
Several requirements should be met by elements and signatures before they can be used in a regional tracer system: the elements should be pollution-derived, sampled and measured accurately, emitted stably and homogeneously in each region, and present on particles small enough to be transported long distances; each signature should remain recognizable during transport. Our preliminary assessment indicates that all these requirements are met adequately; we illustrate several of them in the description that follows. Nevertheless, some of these requirements, such as conservation of proportions during transport, are sufficiently critical that we have built routine checks into our operating system.
In our invention a system in which the relative abundances of selected elements in pollution aerosol (suspended particles) are used to determine the region(s) from which it has originated. The technique can be applied at distances of hundreds to thousands of kilometers from the sources, i.e., in regions ranging from rural to very remote, and where conventional approaches such as emission inventories, air-mass trajectories, and long-range transport models fail. For example, both Canada and the United States now agree that the several long-range transport models currently in use for predicting the origins of sulfate and acidity are unverified and unverifiable in the near future. We thus view our elemental technique as a powerful alternative.
Our invention distinguishes from the known prior art in that we recognize and document the existence of regional elemental signatures. The technique described hereinafter represents our formalized method for developing these signatures and using them quantitatively.
The techniques has several key features. It preferably uses seven pollution-derived, fine-particle elements in the signatures (arsenic, selenium, antimony, zinc, indium, manganese, vanadium). These elements are chosen from the 40 or so that we can measure by neutronactivation analysis because they are the most pollution-derived and are determined best, i.e., have the lowest analytical uncertainties. This list of elements is not static; it can be altered as needed, and should expand in the future as other analytical techniques are employed. For elements such as manganese and vanadium, a substantial fraction of whose mass comes from suspended soil dust, only the pollution-derived component is used. It is important that only fine-particle elements be used in this tracer system, so that they will remain in the atmosphere for long periods and their proportions will not change during transport.
Regional signatures are preferrably built from the six elemental ratios to selenium rather than the seven absolute concentrations. Ratios are used to correct for changes in elemental concentration due to dispersion and removal during transport.
The regional signatures are built up from multiple samples at multiple sites within each region. Because any region can be influenced by aerosol from outside it, only some of the samples from a given region can be used to characterize it. They must be chosen from the total set of samples with care. We generally use some combination of a priori knowledge of the region, modal analysis of the frequency distributions of the elemental ratios, and meterological analysis to do this. To be safe, the signature of a region should not be considered known until samples inside and outside the region agree. From the final set of samples representing a region, geometric means and geometric standard deviations are calculated from each elemental ratio to selenium. The collection of six means and six standard deviations is the elemental signature of that region.
A sample of aerosol from a receptor region can be assigned to a most-likely source area by discriminant analysis, which compares the six elemental ratios to those of samples from possible source regions in a multivariate sense and assigns probabilities that the sample came from each region.
Contributions of several source regions to the signature elements can be determined by least-square apportionment, using various calculational routines and elemental weighting factors.
Contriubtions of several source regions to sulfate aerosol (formed mostly from SO.sub.2 during transport) or other nonsignature constituents can be determined by regressing the regional coefficients of a suite of samples against their sulfate concentrations.
Unique features of our tracer technique are its set of seven elements and its regional approach. Elemental tracers have been used to determine the sources of urban aerosol for several years. Our regional approach, however, uses different elements, a different form for the signatures, builds up the signatures according to a different protocol, and manipulates them with different statistical techniques.
We have determined that well-defined regional signatures exist in both North America and Europe. In North America, we have measured both midwestern and coastal influences on pollution aerosol at Underhill, Vt.; Narragansett, R. I.; High Point, N. J.; and Allegheny Mountain, Pa. The meterology of many of the samples in question was sufficiently obscure that it could not have revealed their origins with any confidence. In Vermont, the majority of the sulfate aerosol in summer comes from the Midwest; in Rhode Island, it comes from the Northeast. We have also detected a signature from the nonferrous smelters of the Sudbury Basin and shown that their contributions to sulfate aerosol in New England during summer are small. The role of these smelters has been discussed for years; we have provided the first direct answers. In Europe, we have shown that the aerosol of a particularly strong pollution episode in Sweden and Finland came from East Europe, not West Europe or the United Kingdom. In the Arctic, we have shown that aerosol at both Barrow, Alaska and Bear Island (Norwegian Arctic) is dominated by Eurasian rather than North American sources. At each site, aerosols from different parts of Eurasia have been detected.